Method for generating a data logout in a computing environment

ABSTRACT

A method and system are disclosed for generating a data logout in a computing environment having a processor and an input/output subsystem for receiving data from and sending data to the processor. The method comprises the steps of the input/output subsystem maintaining a defined set of data; at defined time, the processor issuing a data logout command; and sending said data logout command to the input/output subsystem. In response to receiving said data logout command, the input/output subsystem sends said defined set of data to a specified location. In a preferred implementation, the input/output subsystem includes a channel subsystem and a plurality of input/output devices, and the data logout command is sent to one of the input/output devices. Also, in the preferred embodiment, the channel subsystem includes a channel control unit function and the data logout command is sent to the channel control unit function.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to commonly-owned, U. S. patentapplication Ser. No. (POU920070141US1 (21254-1)), for “SYSTEM FORGENERATING A DATA LOGOUT IN A COMPUTING ENVIRONMENT” filed concurrentlywith this application, the whole contents and disclosure of which isexpressly incorporated by reference herein as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to computing environments, and morespecifically, the invention relates to data logout procedures incomputing environments. Even more specifically, the preferred embodimentof the invention relates to a method and system well suited forgenerating a data logout command across a channel-to-channel connectionin a computing environment.

2. Background Art

Channel-to-channel (CTC) adapters have been used for many years as ageneral-purpose communication mechanism between computer systems. Forexample, CTC adapters have been the principle mechanism for connectingmultiple S/390 system and zSeries (offered by International BusinessMachines Corporation of Armonk, N.Y.) hosts together in homogeneousenvironments. S/390 and zSeries hosts can also be connected to otherheterogeneous environments, such as IBM's RS/6000 and/or AS/400 systems.The CTC adapter is independent of the end users protocol, and has wideapplication in areas such as coupling of multiprocessor systems as wellas in traditional communications protocol stacks (e.g., TCP/IP, SNA).

The CTC adapter provides a CTC control unit (CU) function within achannel residing in a channel subsystem. The CTC CU function providescontrol of CTC I/O devices. I/O device types include such components asperipheral cache memories, data storage units such as direct accessstorage devices (DASD), printers, tape recorders, and the like. From aprogramming viewpoint, control of a CTC I/O device is accomplishedthrough a logical device (simply “device” hereinafter), each residing ina channel subsystem and associated with a CTC CU function.

When an I/O device is controlled through commands passing from a channelin one channel subsystem to a channel in another channel subsystem via aCTC CU function, the link between the channels is a CTC connection (orchannel path) comprising one or more logical paths. When a system firstbecomes operational, typically at least one logical path is establishedand one or more CTC connections are initialized, thus allowing I/Ocommands to control devices.

One widely used channel-to-channel connection is the IBM Fiberconnection, known more commonly as FICON, which provides high speedlinks used for attaching, for example, s/390 control units to s/390systems, allowing interconnection of input/output (I/O) devices and hostsystems that are separated by tens of kilometers. For example, FICON maybe used with the IBM Enterprise Storage Server. FICON is based on theindustry-standard Fibre Channel architecture; consequently, the two havemuch in common. FICON is described in detail by C. M. DeCusatis, et al.,in “Fiber optic interconnects for the IBM S/390 Parallel EnterpriseServer GS, ” IBM Journal of Research and Development, Vol. 43, No. 5/6(September/November 1999), pp. 807 828. IBM, FICON, IBM EnterpriseStorage Server, and S/390 are trademarks of the International BusinessMachines Corporation, Armonk, N.Y.

FICON supports several topologies, including point-to-point andswitched. In a switched topology, one link is connected from a FICONchannel to a port on a switch, and another link is connected from a porton a FICON control unit to port on the same switch. Typically, switchedtopology is used to connect two or more hosts to two or more controlunits such as I/O devices.

FICON has proven to be a very successful and is widely used. It isnonetheless believed that FICON can be improved. For instance, it may bedifficult to debug FICON channel-to-channel (FCTC) devices because twochannels are involved in a single operation, and if is possible for theFCTC control unit functionality for the operation to be on either of theinvolved channels. The channel that possesses the control unitfunctionality is where decisions are made relative to what the activestate is and how commands are answered. Logging the state of the channelwith the CU function is advantageous to more expeditious debug of FCTCproblems. While a mechanism exists (although not yet exploited by z/OS)to force a non-disruptive channel log on the local channel via a CHSCinstruction, no such means exists to signal the attached channel to logout if the FCTC control unit function is across a physical link.

SUMMARY OF THE INVENTION

An object of this invention is to improve data logout procedures incomputing environments.

Another object of the present invention is to provide a procedure forlogging data out from the channels of a channel-to-channel connection ina computing environment.

A further object of the invention is to signal across the link, of achannel-to-channel connection, to the remote channel to create a logout.

These and other objectives are attained with a method and system forgenerating a data logout in a computing environment having a processorand an input/output subsystem for receiving data from and sending datato the processor. The method comprises the steps of the input/outputsubsystem maintaining a defined set of data; at defined time, theprocessor issuing a data logout command; and sending said data logoutcommand to the input/output subsystem. In response to receiving saiddata logout command, the input/output subsystem sends said defined setof data to a specified location.

In a preferred implementation, the input/output subsystem includes achannel subsystem and a plurality of input/output devices, and the stepof sending the data logout command to the input/output subsystem includethe step of sending the data logout command to one of the input/outputdevices. The input/output devices may be embedded in the channelsubsystem; however, the present invention can also be used withinput/output devices that are not embedded in the channel subsystem.Also, in this preferred implementation, said defined set of datadescribes the states of one or more of said input/output devices. Inaddition, in the preferred embodiment, the channel subsystem includes achannel control unit function for controlling the operation of saidinput/output devices; and the step of sending the data logout command tothe input/output subsystem further includes the step of sending the datalogout command to the channel control unit function.

Also, in the preferred embodiment, the step of the processor issuing thedata logout command includes the step of the operating system of theprocessor issuing said data logout command. Further, in a preferredembodiment, the processor receives an error indication, and theprocessor analyzes this error indication and issues the data logoutcommand if the error indication meets one or more predeterminedconditions.

The preferred embodiment of the invention is well suited for use in acomputing environment having channel-to-channel connections, where thechanneling subsystem includes a first channel local to the processor anda second channel remote from the processor. In this preferredembodiment, regardless of which of these channels the channel controlunit function is on, the invention effectively logs out the desired datain response to the data logout command.

The preferred embodiment of the invention, described below in detail,provides a new FICON CTC supported ‘sense-type’ command that will causethe FCTC control unit host channel to take a non-disruptive log,regardless of whether that control unit function resides on the localchannel or across a physical link. The command will be invoked out ofthe FCTC device ERP whenever a diagnostic bit has been set via a DIAGxxPARMLIB member under the control of Product Engineering or Level 2service. This allows a hardware error log to be obtained at the instantsoftware detects an unusual condition and enters its error recoveryprocedure and from whichever channel the FCTC control unit functiondecision-making is occurring on. This hardware error log may be used tohelp correlate the channel hardware perspective at the time of the errorwith the software data logged in EREP.

Further benefits and advantages of the present invention will becomeapparent from a consideration of the following detailed description,given with reference to the accompanying drawings, which specify andshow preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a computing environment wherein twochannels are linked by a channel-to-channel connection and in which thepresent invention is practiced.

FIG. 2 is a block diagram showing in more detail the logical pathsbetween the channels shown in FIG. 1.

FIG. 3 shows the implementation of a channel-to-channel debug command inaccordance with a preferred embodiment of the present invention.

FIG. 4 shows a General Node Element Qualifier modified to include adiagnostic command in accordance with a preferred embodiment of thisinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiment of the present invention provides a way ofissuing a non-disruptive logout command across the link between channelsof a channel-to-channel connection in a computing environment. Theinvention may be employed in a computing environment based on, forexample, one or more zSeries 900 computers offered by InternationalBusiness Machines Corporation, Armonk, N.Y. CTC connections in a zSeries900 environment are described in an IBM publication entitled “ESCON andFICON Channel-to-Channel Reference,” IBM Publication No. SB10-7034-01,Second Edition, October 2001, which is hereby incorporated herein byreference in its entirety. Further, the computing environment mayinclude, for instance, at least one processor or central electroniccomplex (CEC) having one or more logical partitions and one or morechannels.

By way of example, FIG. 1 depicts one embodiment of a computingenvironment 100 having a processor A 102 and a processor B 104, whichincludes a channel A 106 and a channel B 108, respectively. A logicalpath is established over physical link 110 (hereinafter, simply logicalpath 110) that links channel A 106 and channel B 108 via fibre channel(FC) switch 112. Via communication links, channel A is shared by logicalpartitions LP1A 114, LP2A 116 and LP3A 118. Similarly, logicalpartitions LP1B 120, LP2B 122, LP3B 124 share channel B. Alternatively,channel A and/or channel B could be unshared, in which case only onelogical partition would have a link to the channel.

A more detailed embodiment of the channels within a computingenvironment, generally denoted 200, is depicted in FIG. 2, wherein afirst computing system has a first channel 202 and a second computingsystem has a second channel 204 which are linked via a FC switch 206.Each channel includes a channel function 208, 210 and channel 202includes an integrated CTC CU function 212. CTC CU function 212comprises a control unit, which coordinates communication between thechannel functions within the different channels. The CTC CU function isdescribed in the IBM publication entitled “ESCON and FICONChannel-to-Channel Reference,” IBM Publication No. SB10-7034-01, SecondEdition, October 2001.

In FICON architecture, device level communication between a channel anda control unit requires that a logical path be established between them.In the case of CTC communication, two channels each talk to a two-sidedCTC control unit; therefore two logical paths are required for acomplete CTC connection. A local logical path is established across aninternal link between the channel function and the CTC function on thesame channel. This path 214 is shown internal to channel A in FIG. 2.The remote logical path is established across the Fibre Channel link.This path 216 is shown connecting the two channels through FC switch 206in FIG. 2.

Both channels 202, 204 have the capability to include an integrated CTCCU function and can be, for instance, FICON channels offered byInternational Business Machines Corporation. In environments such as theFICON computing environment shown in FIG. 2, the CTC CU function canswitch from one channel to the other. This novel switching capability isdescribed in U.S. Pat. No. 6,728,772 entitled “Automatic Configurationof a Channel-to-Channel Connection Employing Channel-to-ChannelFunctioning Integrated Within One or More Channels of a ComputingEnvironment”. This switching of the CTC CU function can occur aftervarious events, including a re-initialization of the CTC connection.When the CTC connection is initialized or reinitialized, aload-balancing algorithm determines which end of the connection willprovide the CTC CU function. The load-balancing algorithm is describedin the above-mentioned U.S. Pat. No. 6,728,772.

In system 100, the processors send data to and receive data from aninput/output system. The input/output system includes a channelsubsystem and a plurality of input/output devices. Those input/outputdevices may be embedded in the channel subsystem; however, thisinvention can be used with input/output devices that are not embedded inthe channel subsystem.

When an error or fault condition is detected in or in the operation ofthe channel-to-channel connection, the states or conditions of variousdevices are recorded, and this recording is referred to as a snapshot.These conditions form a log, and these conditions or states may bereported to the operating system of one of the processors, a procedurereferred to as logging out or a log out. These logs may then be analyzedby the operating system to determine and to correct the condition thatcaused the error. The logs may be used for other purposes as well,including statistical analysis.

In the operation of the channel-to-channel connections, in the absenceof a hardware detected error or fault condition, it can be difficult toisolate a FICON channel-to-channel (FCTC) problem to the application orthe firmware used with the channels. This is due, in part, to the factthat firmware errors often surface as software detected conditions. Ifthis happens, no channel log is taken, since firmware is unaware of itsfault. This makes debugging of application-detected errors difficult andtime-consuming. Manual initiation of a log from the SE panel can bedone, but this is slow and can, at best, occur seconds after the errorhas become apparent to the operator.

The FCTC Control Unit (CU) function maybe resident on the channel at theother end of the fiber link from where an error is detected. However,the channel that possesses CU functionality is where decisions are maderegarding how to answer commands based on the state of the device. ThisCU functionality can be resident in the local channel or a channelacross a physical link, and potentially in a different physical system.The preferred embodiment of the present invention provides a real-timemechanism to signal across the link to the remote channel to create alogout.

More specifically, the preferred embodiment of the invention provides adebug command that can be used to signal firmware immediately upondetection of an error condition in software. This debug command can beissued directly out of the error recovery process (ERP) of the operatingsystem of one of the processors 102, 104 to cause a non-disruptive logto be taken by the channel in which the FCTC CU functionality resides.The use of this debug command can be limited to control of the serviceteam, or for specific error conditions in order to avoid excessivelogouts.

For example, a returned one-byte of data can be used to correlatechannel logs with a software I/O trace to identify instances whenmultiple logs arc initiated. An additional mechanism that can allow forobtaining a log from the remote channel with the FCTC function isresident on the channel local to where the command was issued.

The debug command can be issued by the operating system of eitherprocessor 102, 104 upon detection of an error condition in software, andthe debug command causes a non-disruptive channel log to be taken whilenot altering the existing states of the devices. Any resetting eventcondition instructions, if existing, remain pending. Also, preferably,any previously owed status is not presented in response to the debugcommand. The debug command executes successfully (CE/DE status)regardless of such state, and requires only that paths be established tothe FCTC device.

Preferably, the debug command is used prior to normal processing of thedetected error since the device state is preserved across execution ofthe command. Further, preferably, die debug command is a “sense” typecommand, and the command returns a single byte sequence number that canbe used for correlation of channel logs with software GTF traces.

FIG. 3 outlines a procedure for implementing an embodiment of thepresent invention. At step 310, a status information unit (IU) is sentfrom one of the channels of the channel-to-channel connection to the ERPof the operating system of one of the processors 102, 104. At step 320,that ERP determines whether the status information unit is bad or has aspecified condition code. The ERP also, at step 330, examinesself-description data in the status IU; and at step 340, the ERPdetermines if the debug command is supported by the operating system.

If this function is supported, then the debug command is sent to theFCTC channel that sent the status IU. This command, at step 350, forcesa non-disruptive log—that is, a record of the states of various,specified devices is sent to the operating system. Also, at step 350,channel end and device end signals arc returned to the operating systemERP. If, at step 340, the debug command is not supported by theoperating system, then at step 360, the error condition identified inthe Status IU sent to the ERP at step 310 is processed using the normalERP procedure.

FIG. 4 shows a General Node Element Qualifier (GNEQ) 400 that, in thepreferred embodiment of the invention, stores the debug command 410.Support for this diagnostic command is indicated in the self-descriptiondata, in word 2 of the GNEQ. This word, previously, was a reservedmodel-dependent area. A flag bit, stored at 420, indicates that commandsupport is provided along with the one-byte op-code of the command.

In addition, preferably, the debug command of the present inventionprovides interoperability. A new debug command issued to a channel thatdoes not support the command, results in a unit check-sense data equalcommand reject.

The present invention results in logging across the link betweenconnected channels. As mentioned above, the FCTC function can reside onthe channel local to where the command is issued or across a physicallink to a remote channel. When the debug command is received by the FCTCfunction that resides locally to the channel that issued the command,the FCTC function will cause an error condition to occur on the physicalinterface to the remote channel, and this will, in turn, cause thatremote Channel to log out as well. For example, the FCTC function maygenerate a link error by sending an unsolicited status frame with badLRC. This prevents association of the link error with a specific deviceand generates an IFCC log, which by design docs not “call home” forservice action.

An additional aspect of this invention is the method by which the‘processor’ (actually the operating system running on the processor)discovers the command code to use when the debug, or logout, command issent to the device. During initialization, the processor gathers ‘selfdescription’ data from all the I/O devices in the processor'sconfiguration. This data contains relevant characteristics for eachdevice, such as the device's serial number, plant of manufacture, etc.Part of the self-description data gathered during this process is theGNEQ described above and shown in FIG. 4. For this aspect of theinvention, a debug command code field and a validity bit are added tothe GNEQ.

When, during normal operation, the operating system of the processor gotinitiative to generate a logout for a particular device, that operatingsystem would look up the command code in the saved GNEQ for that device,and use that command code to build and send the debug command to thedevice. This way, any given device can pick whatever command code thedevice wanted to assign for this purpose, rather than dictating aspecific code which must be used by all devices. This is much moreflexible, since for example, an unused command code which might bepicked for CTC might already be assigned to another function in otherdevice types such as disk or tape.

Although various embodiments are described above, these are onlyexamples. Processing environments other than those described herein, canincorporate and use one or more aspects of the present invention.Further, although various control blocks have been shown, the locationof the information within those control blocks may be other than shownherein. Further, each control block may include additional, less ordifferent information than described herein. For instance, there may beadditional, fewer and/or different fields, including fields that mayinclude additional, fewer and/or different flags. Further, there may beadditional, fewer and/or different field sizes. Yet further, althoughmain memory may be used in various portions of the embodiment, one ormore aspects of the present invention may be applicable to other memory.Still further, although communications protocols, such as FICON and FCP,are described herein, one or more aspects of the present invention areapplicable to other protocols.

Moreover, an environment may include an emulator (e.g., software orother emulation mechanisms), in which a particular architecture orsubset thereof is emulated. In such an environment, one or moreemulation functions of the emulator can implement one or more aspects ofthe present invention, even though a computer executing the emulator mayhave a different architecture than the capabilities being emulated. Asone example, in emulation mode, the specific instruction or operationbeing emulated is decoded, and an appropriate emulation function isbuilt to implement the individual instruction or operation.

In yet other examples, TCW channel programs performing other commandsmay include one or more aspects of the present invention. Further, TCWchannel programs can read other amounts of data than described hereinand still benefit from one or more aspects of the present invention.Numerous other examples and modifications are possible without departingfrom the spirit of the present invention.

In an emulation environment, a host computer includes, for instance, amemory to store instructions and data; an instruction fetch unit tofetch instructions from memory and to optionally, provide localbuffering for the fetched instruction; an instruction decode unit toreceive the instruction fetch unit and to determine the type ofinstructions that have been fetched; and an instruction execution unitto execute the instructions. Execution may include loading data into aregister for memory; storing data back to memory from a register; orperforming some type of arithmetic or logical operation, as determinedby the decode unit. In one example, each unit is implemented insoftware. For instance, the operations being performed by the units areimplemented as one or more subroutines within emulator software.

Further, a data processing system suitable for storing and/or executingprogram code is usable that includes at least one processor coupleddirectly or indirectly to memory elements through a system bus. Thememory elements include, for instance, local memory employed duringactual execution of the program code, bulk storage, and cache memorywhich provide temporary storage of at least some program code in orderto reduce the number of times code must be retrieved from bulk storageduring execution.

Input/Output or I/O devices (including, but not limited to, keyboards,displays, pointing devices, etc.) can be coupled to the system eitherdirectly or through intervening I/O controllers. Network adapters mayalso be coupled to the system to enable the data processing system tobecome coupled to other data processing systems or remote printers orstorage devices through intervening private or public networks. Modems,cable modems, and Ethernet cards are just a few of the available typesof network adapters.

The capabilities of one or more aspects of the present invention can beimplemented in software, firmware, hardware, or some combinationthereof. At least one program storage device readable by a machineembodying at least one program of instructions executable by the machineto perform the capabilities of the present invention can be provided.

The flow diagrams depicted herein are just examples. There may be manyvariations to these diagrams or the steps (or operations) describedtherein without departing from the spirit of the invention. Forinstance, the steps may be performed in a differing order, or steps maybe added, deleted, or modified. All of these variations are considered apart of the claimed invention.

Although preferred embodiments have been depicted and described indetail there, it will be apparent to those skilled in the relevant artthat various modifications, additions, substitutions and the like can bemade without departing from the spirit of the invention and these aretherefore considered to be within the scope of the invention as definedin the following claims.

1. A method of generating a data logout in a computing environmenthaving a processor and an input/output subsystem for receiving data fromand sending data to the processor, the method comprising the steps of:the input/output subsystem maintaining a defined set of data; at definedtimes, the processor issuing a data logout command; sending said datalogout command to the input/output subsystem; and in response toreceiving said data logout command, the input/output subsystem sendingsaid defined set of data to a specified location; and wherein: theinput/output subsystem includes a channel subsystem including a firstchannel local to the processor, and a second channel remote from theprocessor, wherein the first and second channels are connected togetherby a channel interface; the step of sending said defined set of dataincludes the steps of, in response to receiving said data logoutcommand, the input/output subsystem sending a specified signal acrosssaid interface to one of said first and second channels; and in responseto receiving said specified signal, said one of the first and secondchannels sends a subset of the defined set of data to the specifiedlocation.
 2. A method according to claim 1, wherein: the input/outputsubsystem further includes a plurality of input/output devices; and thestep of sending the data logout command to the input/output subsystemincludes the step of sending the data logout command to one of theinput/output devices.
 3. A method according to claim 2, wherein saiddefined set of data describes the states of one or more of saidinput/output devices.
 4. A method according to claim 2, wherein: thechannel subsystem includes a channel control unit function forcontrolling the operation of said input/output devices; the step ofsending the data logout command to the input/output subsystem furtherincludes the step of sending the data logout command to the channelcontrol unit function; and the step of the input/output subsystemsending the specified signal includes the step of the channel controlunit function sending the specified signal across said interface.
 5. Amethod according to claim 1, wherein the processor has an operatingsystem, and wherein: the step of the processor issuing the data logoutcommand includes the step of said operating system issuing said datalogout command.
 6. A method according to claim 1, further comprising thesteps of: the processor receiving an error indication; and wherein: thestep of the processor issuing the data logout command includes the stepsof: the processor analyzing said error indication, and the processorissuing the data logout command if the error indication meets one ormore predetermined conditions.
 7. A method according to claim 6,comprising the further step of the processor invoking a defined errorrecovery procedure at given times after receiving said error indication,including the step of the processor invoking said error recoveryprocedure after the step of issuing the data logout command.
 8. A methodaccording to claim 7, wherein: the step of the input/output subsystemsending said defined set of data includes the step of the input/outputsubsystem sending said defined set of data to the processor: and thestep of the processor invoking said error recovery procedure includesthe step of the processor invoking said error recovery procedure afterreceiving said defined set of data from the input/output subsystem.
 9. Amethod of generating a data logout in a computing environment having aprocessor and an input/output subsystem for receiving data from andsending data to the processor, the method comprising the steps of: theinput/output subsystem maintaining a defined set of data; at definedtime, the processor issuing a data logout command; sending said datalogout command to the input/output subsystem; and in response toreceiving said data logout command, the input/output subsystem sendingsaid defined set of data to a specified location, wherein: theinput/output subsystem includes a channel subsystem and a plurality ofinput/output devices; and the channel subsystem includes a first channellocal to the processor, and a second channel remote from the processor;the step of the input/output subsystem maintaining the defined set ofdata includes the steps of the first channel maintaining a first subsetof the defined set of data; the second channel maintaining a secondsubset of the defined set of data; and the step of sending said definedset of data includes the step of sending the first and second subsets ofthe defined set of data to the specified location.
 10. A methodaccording to claim 9, wherein: the step of sending the data logoutcommand includes the step of sending the data logout command to one ofsaid input/output devices; and the step of sending said first and secondsubsets of the defined set of data further includes the step of sendingsaid first and second subsets of the defined set of data in response tosaid one of the input/output devices receiving the data logout command.11. A method according to claim 9, wherein the first and second channelsare connected together by a channel interface, and the step of sendingthe data logout command includes the step of sending the data logoutcommand to the first channel.
 12. A method according to claim 11,wherein the step of sending the defined set of data includes the stepsof: the first channel causing an error condition on said interface; andthe second channel sending said second subset of the defined set of datain response to said causing the error condition on said interface.
 13. Amethod according to clam 9, wherein the channel subsystem furtherincludes a channel control unit function for controlling saidinput/output devices.
 14. A method according to claim 13, wherein thechannel control unit function is on the first channel local to theprocessor.
 15. A method according to claim 13, wherein the channelcontrol unit function is on the second channel remote from theprocessor.
 16. A method of deploying a computer program product forgenerating a data logout in a computing environment having a processorand an input/output subsystem for receiving data from and sending datato the processor, said method comprising the steps of: maintaining adefined set of data on the input/output subsystem; issuing a data logoutcommand from the processor at defined times; sending said data logoutcommand to the input/output subsystem; and sending said defined set ofdata to a specified location in response to the input/output subsystemreceiving said data logout command; and wherein: the input/outputsubsystem includes a channel subsystem including a first channel localto the processor, and a second channel remote from the processor,wherein the first and second channels are connected together by achannel interface; the step of sending said defined set of data includesthe steps of, in response to receiving said data logout command, theinput/output subsystem sending a specified signal across said interfaceto one of said first and second channels; and in response to receivingsaid specified signal, said one of the first and second channels sends asubset of the defined set of data to the specified location.
 17. Amethod according to claim 16, wherein: the input/output subsystemfurther includes a plurality of input/output devices; and the step ofsending the data logout command includes the step of sending the datalogout command to one of the input/output devices.
 18. A methodaccording to claim 17, wherein: the channel subsystem includes a channelcontrol unit function for controlling the operation of said input/outputdevices; step of sending the data logout command includes the step ofsending the data logout command to the channel control unit function;and the step of the input/output subsystem sending the specified signalincludes the step of the channel control unit function sending thespecified signal across said interface.
 19. A method of issuing a datalogout command in a computer environment including a processor and aninput/output system having a plurality of input/output devices, themethod comprising the steps of: the processor gathering self-descriptiondata from all of said input/output devices; the processor, at definedtimes, generating a data logout command for a specified one of theinput/output devices, including the step of using the self descriptivedata from said specified one of the input/output devices to build thelogout command; and sending the data logout command to said specifiedone of the input/output devices; and wherein: the input/output subsystemincludes a channel subsystem including a first channel local to theprocessor, and a second channel remote from the processor, wherein thefirst and second channels are connected together by a channel interface;the step of sending said defined set of data includes the steps of, inresponse to receiving said data logout command, the input/outputsubsystem sending a specified signal across said interface to one ofsaid first and second channels; and in response to receiving saidspecified signal, said one of the first and second channels sends asubset of the defined set of data to the specified location.
 20. Amethod according to claim 19, wherein: each of the input/output deviceshas an associated data logout command; and the gathering step includesthe step of the processor gathering the associated data logout commandsfrom all of the input/output devices.